ESC 350

Midterm II

Winter 1999

 

Section I

1 Sink Habitat

  1. A sink habitat is a place where animals are unable to offset mortality with births (lamda<1). Animals would cease to exist in this habitat without immigration from more productive habitats.
  2. A woodpecker requires 3 ha territories with a fungus-infected nest tree. Once the available territories are taken in the source habitat (N=K where lambda>1), breeder must disperse to other habitats. Continued dispersal to poor breeding habitat can lead to a stable population maintained only through immigration.
  3. If the surplus in the source exceeds the deficit in the sink, animal density in the sink can exceed that in the source. Assuming a positive relationship between density and habitat suitability could be disastrous in the aforementioned case. A manager could protect a source rather than the sink and effect local extinction.

2 Compensatory Mortality

  1. The process that maintains constant annual mortality by compensating for a decrease in one source of mortality with an increase in one or more other sources of mortality.
  2. Lots of fawns are produced annually. If winter conditions are relatively mild in the late summer and fall, more fawns will survive into the winter. However, the number of fawns surviving to one year of age remains similar to that in years of bad fall weather because higher fawn survivorship decreases the available forage, increases food stress, and increases juvenile mortality in the winter.
  3. Managers often rely on assumptions or studies showing that mortality from hunting is compensatory, as opposed to additive, when deciding how many permits to issue.

3 Allometry

  1. The scaling of a variable to an animals body size.
  2. The length of time between a cycling population’s peaks is positively related to the animal’s body mass.
  3. Since general allometric relationships are known (e.g. cycle lengths), a manager who knows his animal’s body size can predict a great deal that animal.

4 Intraspecific Competition

  1. Intraspecific competition is exploitative or interference competition between members of the same species (conspecifics).
  2. Steinwasher showed that tadpole of a certain age produce a toxin that inhibits the progression of other tadpoles (same spp.) toward metamorphosis. This action increases the odds that the older tadpoles will reap the rewards of earlier metamorphosis…more food, mates, etc.
  3. This type of competition often helps set a habitat’s carrying capacity. As the density of animals increases so too does the frequency/severity of competitive interaction. Ultimately, the effects of these interactions decrease births and increase mortality to the point where these two parameters just offset each other…K. Also, if intraspecific competition is stronger than interspecific competition, two species are more likely to coexist.

5 Density-independent Factor

  1. A density-independent factor limits a population in a manner that is unrelated to the population’s density.
  2. Thrips on roses experience a population crash every year when those flowers dry up. The thrip population falls to nearly 10 individuals at this time regardless of the preceding density.
  3. Managers have little to no control over density-independent factors. However, knowledge of their existence allows managers to increase the accuracy of population projections by considering their effects.

Section II

1 Should hungry animals ever ignore abundant food?

Yes. When the return for the effort falls below some threshold because energy and time expended in handling offsets the energy gained from nutrition, the animal should ignore that food item. Essentially, its more profitable to look for another item than waste time on such a poor investment. Think about trying to meet your daily needs scrounging for rice cakes and corndogs that are dispersed across campus. Those who eat rice cakes wouldn’t fare too well.

2 Pianka’s model [dN/dt = (P)(N)(K/N)]

a- The model simplifies to dN/dt = PK. Since N drops out, the rate of population change is not a function of density (actually, abundance as its written).

b- No. Regulation is the return of a population to K due to the influence of a density-dependent factor. Since, the rate of population change equals the product of predation intensity and carrying capacity, regulation won’t occur; the rate will increase as P increases unless there is an offsetting decrease in K.

3 Lotka-Volterra Figure

The figure illustrates the outcome of interspecific competition between two species. Since interspecific competition is weak relative to intraspecific competition (alpha is small, making carrying capacity divided by alpha larger than carrying capacity on same axis), any combination of species 1 and 2 will progress toward a stable equilibrium. This information would be useful if you wanted to predict whether two species might be able to coexist in your wildlife refuge.

4 Logistic Growth Figure

  1. Assuming that the population is above 500, the answer is (N - 500). In other words, reducing the population to 500 animals allows for maximum growth rate and more individuals for harvest.
  2. K=1000, the point where 1) N does not equal zero and 2) births just offset deaths (dN/dt = 0).
  3. The yield can be the same for two population sizes because one has fewer individuals breeding at a faster rate and the other has more individuals breeding at a slower rate.

5 Storing Energy

Bears put on fat instead of muscle (protein) when they hibernate because they need to maximize the amount of energy that they can get from stored tissue. Fat yields more energy per unit mass than protein.